A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to ensure satisfactory operation of the photolithographic apparatus, high quality optical elements such as refractive and/or reflective lenses should be used. However, it has been found that operation of a photolithographic apparatus may have a damaging effect on the optical elements. This is particularly the case when EUV radiation is used for exposing substrates. At EUV radiation wavelengths, satisfactory transparent materials have not not been found. This implies that mainly reflective optical elements (mirrors) should be used and that open connections should be used between different parts of the photolithographic apparatus along the beam path. Use of open connections entails the risk that contaminants from the radiation source or the substrate may reach the optical elements, thereby reducing their optical quality. To prevent this, a gas flow is typically introduced into the path of the beam to drag away contaminant particles before they can reach the optical elements. In the prior art, a heavy gas like argon is typically used, because a heavy gas more effectively drags along particles. As another measure, the use of a foil trap has been described in European Patent Application No. 1 491 963.
However, the added gas flow itself may become a source of danger to the optical elements. When EUV is transmitted through this gas medium, the absorption of EUV light (or other ionizing light) ionizes the gas and a plasma is formed. Because of the higher mobility of electrons, the plasma potential tends to rise above the potential of walls and optical elements, and ions may be accelerated towards walls and optical elements, thereby giving rise to a sputtering effect.